In the security document printing industry, it is commonplace to provide areas on a document with specific optical characteristics which can be identified by illumination in a predetermined way at a particular wavelength and/or polarisation. Examples include the use of holograms and gratings incorporated onto such documents. It has been proposed to incorporate in or on such a document one or more regions of an Optically Variable Material, henceforth referred to as OVM.
An optically variable material (OVM) can comprise any material which changes colour depending on the angle at which a surface containing the material is viewed, and includes pearlescent materials, iridescent materials, liquid crystalline materials and OV inks such as sold under the trade name OVI by Sicpa SA of Lausanne, Switzerland.
It is a property of an OVM that a region printed or coated using such a material will appear differently coloured depending on the viewing angle and the angle of illumination. In one example if a flat printed surface containing one particular OVM is illuminated by white light at an angle of 45° to the normal, the OVM appears purple to the human eye when back scattered light is viewed, orange when direct (specular) reflection is viewed, and green if viewed along the surface of the paper, that is at a glancing angle.
OVM inks generally fall into two categories, non-pearlescent and pearlescent. The optical characteristics of such inks are different and both again differ from the optical characteristics of a non-OVM (metallic) ink.
It is an object of the present invention to provide a method and apparatus for detecting the presence of an OVM in or on a surface, typically the surface of a security document.
It is another object of the present invention to provide a method and apparatus for detecting, and identifying the type of, OVM in or on a surface, typically the surface of a security document.
It is another object of the present invention to provide a method and apparatus for illuminating, and responding to reflected light from, a surface, and which is adapted to generate a signal indicative of material present in or on the illuminated surface.